Typically, floating screed pavers comprise a self-propelled paving vehicle having a hopper at its forward end for receiving material from a dump truck pushed forwardly along the roadbed by the paver. The truck progressively dumps its load of paving material into the hopper.
A conveyor system on the paver transfers the paving material from the paver hopper rearwardly for discharge onto the roadbed in front of transversely arranged screw augers which spread the material laterally in front of the main screed. This screed is commonly operated so as to be called a "floating screed" by being connected to the paving vehicle by pivoted tow arms.
The screed functions to level and compact the paving material distributed by the augers, ideally leaving the finished road with a uniform, smooth surface. The height of the tow points on each side of the paver and the angle of attack of the screed may both be varied to control the depth and surface elevation of the paving mat, all as is well known to those skilled in the art.
For many paving activities, there is a need to widen the effective width of the screed. This has been accomplished by providing either a fixed-width screed with bolt-on fixed-width extensions or by providing the main screed with one or more, usually two, adjustable extensions. These "extendable" screeds have an advantage in efficiency on many paving projects where the paving width varies and/or there are obstacles to be paved around.
Earlier attempts to overcome the problems and inefficiencies involved with trying to pave variable width areas with a fixed width screed led to the development of "strikeoffs"--extendable attachments for mounting, on the front plate of a fixed main screed. Strikeoffs, as they are known in the industry, are characterized by having a sole plate (i.e., the bottom surface of the screed which contacts the paving material) which is substantially narrower (in the direction of travel) than the main screed sole plate as described on U.S. Pat. No. 4,818,140. The related front-mounted extender screed (as described on U.S. Pat. No. 4,379,653) is characterized by having a sole plate of approximately the same width, in the direction of travel, as the main screed sole plate.
Misalignment or deflection of the extender screed and its sole plate, caused by clearance or deflection at the extender or strikeoff mounting and the force of the asphalt or similar paving material on the front of the screed as the screed is towed forward by the paver, results in a mismatch between the sole plate of the main screed and the sole plate of the extension screed and a subsequent unevenness, non-uniformity, and poor quality of the newly paved road surface.
The front mounted strikeoff (usually limited to 3' per side extension beyond the main screed) or front mounted extender (sometimes extended to 8'-9' per side) often rely on the strikeoff or extender deflecting rearwardly into contact with the main screed as a means of limiting the misalignment between main and extender screed sole plates. This however still often leaves an unacceptable level of deflection and a poor quality road surface, especially on paving projects where the total extended screed width may reach to 26 feet or more. The front mounted extender at wide widths also exhibits the problem of difficulty feeding paving material to the outer ends of the screed extension.
Rear mounted extenders (mounted rearwardly of the main screed), offer the possibility of a more rigid mounting of the screed extensions and less deflection under load. Rear mounted extenders have other benefits such as improved feeding at wide widths as the paving material cascades more naturally outward and to the rear as the screed is towed forward by the paver. Rear mounted extenders, therefore, often make it possible to pave wider widths without adding cumbersome fixed transverse conveyor auger sections, greatly improving the efficiency of the paving operation.
European extendable screeds are typically constructed with extenders mounted rearwardly of the main screed. Rearward of the front face of the main screed there is then the space and structural strength to provide a more rigid extender mounting and extender height adjustment which does not deflect significantly while paving.
In the use of asphalt pavers, particularly in the U.S., there has also been a need not only to extend the width of the screed, but also to form a sloped shoulder or berm at the edge of the road. Thus, the extender screed units are mounted for pivoting movement, in a vertical plane transverse to the direction of travel, in addition to extending and retracting laterally. Prior art extension screed pivot joints generally involve a loose or headed pin or shoulder bolt with some axial clearance in the radial and/or axial clearance direction. Some prior art joints, such as the previously mentioned front-mounted strikeoff mount, lack the structural strength to totally support the extension unit and must rely on rearward deflection of the extension into contact with the main screed to limit misalignment of main and extension sole plates.
The invention described herein seeks to solve the current problems of providing an extension screed unit mounting joint which permits the extension screed and its mounting to be rear mounted, of simple and sturdy construction, extend, retract, and pivot yet maintain zero clearance between the main and extender screed. The result is improved alignment of main and extender screed sole plates while paving and a smoother more uniform paved road surface.
The invention also permits easy access to and adjustment of the extension screed mounting area and pivot. The pivot area may be easily tightened to maintain zero clearance between the main and extension screeds (and sole plate alignment) over the life of the paving machine.